If a flow is created between two stationary areas due to a pressure difference, (theoretically) a symmetrical velocity profile will form (Figure S 101 a), which is caused by adhesion of the fluid to the surface. The flow rate/pressure characteristic is described by Hagen-Poiseuille's law (Circular ring gap, gap seal).
However, if a surface is moved at velocity v (e.g. the piston rod), a Newtonian drag flow will form without a differential pressure with a velocity distribution (b).
These two velocity profiles overlap if the pressure difference acting in the direction of the arrow meet a (piston) movement in the same direction (c) or in the opposite direction (d).
The flow profile (c) is less favourable than (d) for a sealing process.
Figure S 101: Flow in gaps